A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of a die, one die, or several dies) of a substrate (e.g. a silicon wafer). The pattern is typically transferred onto the target portion via imaging onto a layer of radiation sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
A conventional lithographic apparatus comprises an illumination system, a support to hold a patterning device and a projection system. The illumination system is arranged between a radiation source (for example, an extreme ultra-violet radiation plasma source) and the patterning device support. The illumination system is configured to receive radiation from the source, produce a beam of radiation and project the beam of radiation onto the patterning device. The beam of radiation is patterned by the patterning device. The projection system is arranged between the patterning device support and the substrate. The projection system is configured to project the patterned beam of radiation onto the target portion of the substrate.
Stray radiation in a lithographic apparatus may reduce the accuracy with which a pattern is projected onto a substrate.